Apoptosis is the process by which a cell undergoes programmed cell death in response to nutrient deprivation, stress signals, death receptor signaling, DNA damage, treatment with novel targeted or cytotoxic agents or other insults from the external environment. Two forms of apoptosis have been identified: the intrinsic or mitochondrial pathway involving members of the BCL2 family of proteins and BH3 only proteins, and the extrinsic pathway where signals from death domain containing receptors trigger the activation of the caspasc cascade via regulation of members of the inhibitor of apoptosis (IAP) family of proteins.
The BCL2 family of BH-3 containing proteins, comprising Bcl-2, Bcl-XL, Mcl-1, Bcl-w and Bcl-A1 (also known as Bfl-1), is a family of adaptor molecules involved in regulating the control of mitochondrial apoptosis in a variety of different cell types (reviewed in (1)). BCL2 family members are generally considered to be anti-apoptotic because they bind to and counteract the activity of pro-apoptotic members of the BH3-only family, including Bim, tBid, and Puma, and the multidomain effector proteins Bak and Bax. Bim and tBid in turn facilitate the oligomerization and activation of Bak and Bax to form a pore in the outer mitochondrial membrane through which Smac and cytochrome c are released into the cytosol. The release of cytochrome c triggers activation of the caspase cascade via formation of a complex with Apaf-1, termed the apoptosome, ultimately leading to apoptotic cell death. Another group of BH3-only proteins, the “sentinels” are upregulated by a variety of transcriptional and post-translational mechanisms in response to the pro-apoptotic triggers mentioned above. These proteins, including Noxa, Bmf, Bad, Bik, and Hrk bind selectively to certain BCL2 family members and alter the balance of free and bound pro-apoptotic members, through a process of sensitization (binding to anti-apoptotic BCL2 family members) and depression (displacing bound Bim, tBid, Bak and Bax), permitting permeabilization of the outer mitochondrial membrane (MOMP) to occur. In healthy cells, the balance of pro- and anti-apoptotic proteins ensures that apoptosis is held in check until needed.
The anti-apoptotic BCL2 family members are often found to be up-regulated in cancers and have been associated both with stage of disease and prognosis. Over-expression of Bcl-2, Bcl-XL and Mcl-1 has been linked to resistance to common therapeutic agents and strategies that target BCL2 family members can restore sensitivity to cytotoxic agents by reinstating the ability of the tumor cell to undergo apoptosis. A translocation, t(14; 18)(q32; q31) involving Bcl2 and IGH leads to over-expression of the Bcl-2 protein and is commonly found in tumors of hematological origin including non Hodgkin's Lymphomas (2-4). Even in the absence of a translocation, BCL2 family expression is often de-regulated (2, 3, 5-8). Amplifications of Bcl-XL and Mcl-1 are also commonly seen in many tumor types (9-11) for example by activation of NFkB or by suppression of certain microRNAs (12).
In a number of tumors, including chronic lymphocytic leukemia (CLL) (4, 13-15), small cell lung cancer (SCLC) (16), and prostate cancer (17), Bcl-2 expression is an independent indicator of poor prognosis. In other tumor types such as colorectal cancer Bcl-XL expression is linked to grade and stage (18) and in hepatocellular cancer Bcl-XL expression is an independent marker of poorer overall and disease-free survival (19). Mcl-1 expression has also been linked to stage in CLL and to prognosis, for example in myeloma, melanoma, ovarian and gastric tumors (20-22).
Redundancy has been seen among members of the BCL2 family and is believed to account at least in part for resistance to the BH3-mimetic compounds that target Bcl-2, Bcl-XL, Bcl-w, and Bcl-A1, but not Mcl-1 (23-26). Hence, for many cancers, a combination of a selective BH-3 mimetic with another agent that targets the Bim/Noxa/Mcl-1 axis may be desirable to ensure apoptosis induction and tumor regression. Examples of such agents include but are not limited to cytotoxic chemotherapeutics, proteasome inhibitors, EGFR inhibitors, and MEk/ERK pathway inhibitors.    (1) Chipuk J E, Moldoveanu T, Llambi F, Parsons M J, Green D R. The BCL-2 family reunion. Mol. Cell 2010 Feb. 12; 37(3):299-310.    (2) Majid A, Tsoulakis O, Walewska R, Gesk S, Siebert R, Kennedy D B, et al. BCL2 expression in chronic lymphocytic leukemia: lack of association with the BCL2 938A>C promoter single nucleotide polymorphism. Blood 2008 Jan. 15; 111(2):874-877.    (3) Otake Y, Soundararajan S, Sengupta T K, Kio E A, Smith J C, Pineda-Roman M, et al. Overexpression of nucleolin in chronic lymphocytic leukemia cells induces stabilization of bcl2 mRNA. Blood 2007 Apr. 1; 109(7):3069-3075.    (4) Nagy B, Lundan T, Larramendy M L, Aalto Y, Zhu Y, Niini T, et al. Abnormal expression of apoptosis-related genes in haematological malignancies: overexpression of MYC is poor prognostic sign in mantle cell lymphoma. Br. J. Haematol. 2003 February; 120(3):434-441.    (5) Dierlamm J. Murga Penas E M. Bentink S. Wessendorf S. Berger H. Hummel M. Klapper W. Lenze D. Rosenwald A. Haralambieva E. Ott G. Cogliatti S B. Moller P. Schwaenen C. Stein H. Loffler M. Spang R. Trumper L. Siebert R. Deutsche Krebshilfe Network Project “Molecular Mechanisms in Malignant Lymphomas”. Gain of chromosome region 18q21 including the MALT1 gene is associated with the activated B-cell-like gene expression subtype and increased BCL2 gene dosage and protein expression in diffuse large B-cell lymphoma. Haematologica 2008 May; 93(5):688-696.    (6) Gascoyne R D, Adomat S A, Krajewski S, Krajewska M, Horsman D E, Tolcher A W, et al. Prognostic significance of Bcl-2 protein expression and Bcl-2 gene rearrangement in diffuse aggressive non-Hodgkin's lymphoma. Blood 1997 Jul. 1; 90(1):244-251.    (7) Iqbal J, Neppalli V T, Wright G, Dave B J, Horsman D E, Rosenwald A, et al. BCL2 expression is a prognostic marker for the activated B-cell-like type of diffuse large B-cell lymphoma. Journal of Clinical Oncology 2006 Feb. 20; 24(6):961-968.    (8) Kramer M H, Hermans J, Wijburg E, Philippo K, Geelen E, van Krieken J H, et al. Clinical relevance of BCL2, BCL6, and MYC rearrangements in diffuse large B-cell lymphoma. Blood 1998 Nov. 1; 92(9):3152-3162.    (9) Largo C, Alvarez S, Saez B, Blesa D, Martin-Subero J, Gonzalez-Garcia 1, et al. Identification of overexpressed genes in frequently gained/amplified chromosome regions in multiple myeloma. Haematologica 2006 Feb. 1; 91(2):184-191.    (10) Lombardi L, Poretti G, Mattioli M, Fabris S, Agnelli L, Bicciato S, et al. Molecular characterization of human multiple myeloma cell lines by integrative genomics: Insights into the biology of the disease. Genes, Chromosomes and Cancer 2007; 46(3):226-238.    (11) Beroukhim R, Mermel C H, Porter D, Wei G, Raychaudhuri S, Donovan J, et al. The landscape of somatic copy-number alteration across human cancers. Nature 2010 Feb. 18; 463(7283):899-905.    (12) Calin G A, Cimmino A, Fabbri M, Ferracin M, Wojcik S E, Shimizu M, et al. MiR-15a and miR-16-1 cluster functions in human leukemia. Proc. Natl. Acad. Sci. U.S.A. 2008 Apr. 1; 105(13):5166-5171.    (13) Aalto Y, El-Rifa W, Vilpo L, Ollila J, Nagy B, Vihinen M, et al. Distinct gene expression profiling in chronic lymphocytic leukemia with 11q23 deletion. Leukemia 2001 November; 15(11):1721-1728.    (14) Faderl S, Keating M J, Do K A, Liang S Y, Kantarjian H M, O'Brien S, et al. Expression profile of 11 proteins and their prognostic significance in patients with chronic lymphocytic leukemia (CLL). Leukemia 2002 June; 16(6):1045-1052.    (15) Robertson L E, Plunkett W, McConnell K, Keating M J, McDonnell T J. Bcl-2 expression in chronic lymphocytic leukemia and its correlation with the induction of apoptosis and clinical outcome. Leukemia 1996 March; 10(3):456-459.    (16) Ilievska Poposka B, Smickova S, Jovanovska Crvenkovska S, Zafirovska Ivanovska B, Stefanovski T, Petrusevska G. Bcl-2 as a prognostic factor for survival in small-cell lung cancer. Makedonska Akademija na Naukite i Umetnostite Oddelenie Za Bioloshki i Meditsinski Nauki Prilozi 2008 December; 29(2):281-293.    (17) Szende B, Romics I, Torda I, Bely M, Szegedi Z, Lovasz S. Apoptosis, mitosis, p53, bcl (2), Ki-67 and clinical outcome in prostate carcinoma treated by androgen ablation. Urol. Int. 1999; 63(2):115-119.    (18) Zhang Y L, Pang L Q, Wu Y, Wang X Y, Wang C Q, Fan Y. Significance of Bcl-xL in human colon carcinoma. World J. Gastroenterol. 2008 May 21; 14(19):3069-3073.    (19) Nardone G, Rocco A, Vaira D, Staibano S, Budillon A, Tatangelo F, et al. Expression of COX-2, mPGE-synthasel, MDR-1 (P-gp), and Bcl-xL: a molecular pathway of H. pylori-related gastric carcinogenesis. J. Pathol. 2004 March; 202(3):305-312.    (20) Wacheck V, Cejka D, Sieghart W, Losert D, Strommer S, Crevenna R, et al. Mcl-1 is a relevant molecular target for antisense oligonucleotide strategies in gastric cancer cells. Cancer Biology & Therapy 2006 October; 5(10):1348-1354.    (21) Legartova S, Krejci J, Harnicarova A, Hajek R, Kozubek S, Bartova E. Nuclear topography of the 1q21 genomic region and Mcl-1 protein levels associated with pathophysiology of multiple myeloma. Neoplasma 2009; 56(5):404-413.    (22) Shigemasa K, Katoh O, Shiroyama Y, Mihara S, Mukai K, Nagai N, et al. Increased MCL-1 expression is associated with poor prognosis in ovarian carcinomas. Jap. J. Cancer Res. 2002 May; 93(5):542-550.    (23) Hauck P, Chao B H, Litz J, Krystal G W. Alterations in the Noxa/Mcl-1 axis determine sensitivity of small cell lung cancer to the BH3 mimetic ABT-737. Molecular Cancer Therapeutics 2009 April; 8(4):883-892.    (24) Miller L A, Goldstein N B, Johannes W U, Walton C H, Fujita M, Norris D A, et al. BH3 mimetic ABT-737 and a proteasome inhibitor synergistically kill melanomas through Noxa-dependent apoptosis. J. Invest. Dermatol. 2009 April; 129(4):964-971.    (25) Lin X, Morgan-Lappe S, Huang X, Li L, Zakula D M, Vernetti L A, et al. ‘Seed’ analysis of off-target siRNAs reveals an essential role of Mcl-1 in resistance to the small-molecule Bcl-2/Bcl-XL inhibitor ABT-737. Oncogene 2007 Jun. 7; 26(27):3972-3979.    (26) Tahir S K, Yang X, Anderson M G, Morgan-Lappe S E, Sarthy A V, Chen J, et al. Influence of Bcl-2 family members on the cellular response of small-cell lung cancer cell lines to ABT-737. Cancer Res. 2007 Feb. 1; 67(3):1176-1183.